Bug 2010975

Description of problem:
It seems that starts_with(); method from std::string is missing on EL8.4 even when we use --std=c++2a with gcc (or clang). Both compilers supports C++20 but it seems the implementation is missing from libstdc++.

[root@localhost ~]# g++ --std=c++2a starts_with.c 
starts_with.c: In function 'int main()':
starts_with.c:6:19: error: 'std::__cxx11::string' {aka 'class std::__cxx11::basic_string<char>'} has no member named 'starts_with'
  std::cout << var.starts_with("is") << std::endl;
                   ^~~~~~~~~~~

Version-Release number of selected component (if applicable):
gcc-c++-8.4.1-1.el8.x86_64
clang-devel-11.0.0-1.module+el8.4.0+8598+a071fcd5.x86_64
libstdc++-8.4.1-1.el8.x86_64
libgcc-8.4.1-1.el8.x86_64
clang-tools-extra-11.0.0-1.module+el8.4.0+8598+a071fcd5.x86_64
clang-11.0.0-1.module+el8.4.0+8598+a071fcd5.x86_64
gcc-8.4.1-1.el8.x86_64
clang-libs-11.0.0-1.module+el8.4.0+8598+a071fcd5.x86_64
libstdc++-devel-8.4.1-1.el8.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Create an example program with std::string and use the method starts_with();
2. Fail to compile with bundled gcc and libstdc++

Actual results:
Compiler error during compile phase.

Expected results:
Successful compilation.

Additional info:
std::string starts_with() is defined on C++20: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0457r2.html

It should be supported if we have the option to enable --std=c++2a on compilers.

Example code:
#include <string>
#include <iostream>

int main() {
	std::string var = "this is a test";
	std::cout << var.starts_with("is") << std::endl;

	return 0;
}